Concreting and Early Hardening Processes in Monolithic Reinforced Concrete Structures / Procesai, Vykstantys Betonavimo Ir Pradinio Kietejimo Metu Gelžbetoninese Monolitinese Konstrukcijose

The exploitation time and reliability of monolithic reinforced concrete structures largely depend on concreting technology and process influence during concreting and early setting stages. Different types of cracks in monolithic reinforced concrete structures appear due to internal and external effects. Cracks appear when the technology of structure concreting is damaged, when formwork is removed during the further setting and structures loaded period. In order to avoid micro and macro cracks in monolithic structures, it is important to measure the particular setting time moment and technological process moment when stresses that exceed the permissible values appear in concrete. The article analyses the processes that appear when horizontal, sloping and vertical monolithic reinforced concrete structures are concreted. The analysis of concrete mixture pressure on formwork is performed. The pressure which is calculated according to different countries' methodology is different: the smallest pressure is obtained calculating according to the British recommendations, and the largest pressure is obtained according to French CIB recommendations. In Lithuania, it is recommended to follow the German DIN 18218 standard. The balance conditions of concrete mixture concreting on slope surface are described. The main concreting technology parameters and their interaction are analysed; the speed, intensity and time of continuous concreting technology are presented. When the process of continuous concreting is performed, it is necessary to evaluate the interaction and values of parameters properly. Methodical theoretical calculation is presented. Practical solutions for industrial building construction applying the modern sliding formwork technology are presented. The impact of cement type, superplasticizers and temperature over the concrete mixture mobility, changes, fresh concrete structural strength and concrete setting kinetics are analysed. The main characteristics of the initial setting — the beginning of structure formation, when concrete mixture turns into concrete state — is analysed applying the ultrasonic method. The beginning of structure formation influences the regulated time of concrete mixture laying and compaction. The requirements for structural strength (permissible strength limits) and concreting rate (formwork movement) of freshly formed concrete are set when the construction is performed applying the continuous concreting technology method. The analysis is implemented performing the construction of cylindrical sludge tank with slipping formwork. While performing the analysis during concreting, it was stated that the concrete setting kinetics corresponds to the sludge tank concreting rate. The analysis performed after concreting and in 28 days of hardening revealed that there are no surface defects or cracks, and concrete strength exceeds the required sludge tank design strength.